In Parasite Mr. Park says to Ms. Park,
But do you know what's strangest of all
and then leans into her ear and whispers something to her, which we are not told what that is. What does he whisper into her ear?
Isn't that when they are talking about the driver? I thought it was pretty obvious. Right after he whispers in her ear she shrieks and says out loud:
Like methamphetamine? Cocaine?
He obviously whispered to her that he suspects the driver is doing drugs.
Here is a link to the script (page 38 in the pdf)